Physicochemical properties of aerosol were investigated by analyzing
the inorganic water soluble content in PM<sub>2.5</sub> samples
collected in the eastern part of the Po Valley (Italy). In this area
the EU limits for many air pollutants are frequently exceeded as
a consequence of local sources and regional-scale transport of
secondary inorganic aerosol precursors. Nine PM<sub>2.5</sub>-bound
major inorganic ions (F<sup>&minus;</sup>, Cl<sup>&minus;</sup>, NO<sub>3</sub><sup>&minus;</sup>,
SO<sub>4</sub><sup>2&minus;</sup>, Na<sup>+</sup>, NH<sub>4</sub><sup>+</sup>, K<sup>+</sup>,
Mg<sup>2+</sup>, Ca<sup>2+</sup>) were monitored over one year in
three sites categorized as semi-rural background, urban background
and industrial. The acidic properties of the PM<sub>2.5</sub> were
studied by applying the recently developed E-AIM thermodynamic
model 4 (Extended Aerosol Thermodynamics Model). The experimental data were also examined in relation to the
levels of gaseous precursors of secondary inorganic aerosol (SO<sub>2</sub>, NO<sub>x</sub>, NO,
NO<sub>2</sub>) and on the basis of some environmental conditions
having an effect on the secondary aerosols generation
processes. A chemometric procedure using cluster analysis on
experimental [NH<sub>4</sub><sup>+</sup>]/[SO<sub>4</sub><sup>2&minus;</sup>] molar ratio and
NO<sub>3</sub><sup>&minus;</sup> concentration has been applied to determine the
conditions needed for ammonium nitrate formation in different
chemical environments. Finally, some considerations on the secondary
inorganic aerosol formation and the most relevant weather conditions
concerning the sulfate-nitrate-ammonium system were also
discussed. The obtained results and discussion can help in understanding the
secondary aerosol formation dynamics in the Po Valley, which is one of the most
critical regions for air pollution in southern Europe.